The inhibitory effect of raloxifene on lipopolysaccharide-induced nitric oxide production in RAW264.7 cells is mediated through a ROS/p38 MAPK/CREB pathway to the up-regulation of heme oxygenase-1 independent of estrogen receptor

In this study, we demonstrate that raloxifene, a selective estrogen receptor modulator, is a potent inducer of the anti-inflammatory enzyme heme oxygenase-1 (HO-1). In RAW264.7 macrophages, raloxifene induced HO-1 mRNA and protein expression. Pretreatment of ICI182780, an estrogen receptor (ER) antagonist or knock-down of endogenous ERα or ERβ gene by RNA interference failed to reverse raloxifene-mediated HO-1 induction, indicating an estrogen receptor-independent mechanism. Interestingly, the raloxifene-induced HO-1 expression was suppressed by reactive oxygen species (ROS) scavengers, including glutathione, TEMPO, Me(2)SO, 1,10-phenanthroline, or allopurinol. In addition, buthionine sulfoximine, an inhibitor of reduced glutathione synthesis, or Fe2+/Cu2+ ions enhanced the positive effect of raloxifene on HO-1 expression. Consistent with these findings, raloxifene induced production of intracellular ROS and increased xanthine oxidase activity in vitro. Additional experiments revealed the involvement of mitogen-activated protein kinase (MAPK) kinase6 and p38 MAPK in the up-regulation of HO-1 by raloxifene and identified p38 MAPK as a downstream effector of ROS. Furthermore, the ROS-p38 MAPK cascade targeted the transcription factor cAMP-responsive element-binding protein (CREB). Finally, the functional significance of HO-1 induction was revealed by raloxifene-mediated inhibition of inducible nitric oxide synthase expression and nitric oxide production, a response reversed by the inhibition of HO-1 protein synthesis or blockade of p38 MAPK or xanthine oxidase activity. Therefore, identification of ROS-p38 MAPK-CREB-linked cascade as cellular relays in raloxifene-mediated HO-1 expression defines the signaling events that could participate in raloxifene-mediated anti-inflammatory response.

Research highlights
► Raloxifene induces HO-1 expression in an estrogen receptor-independent manner.
► ROS generation is required for raloxifen- induced HO-1 expression.
► The p38 MAPK-CREB-linked cascade lies downstream of ROS in raloxifene signaling.
► HO-1 mediates the inhibitory effect of raloxifene on LPS-induced NO production.